In recent years, the need has arisen to monitor certain operating conditions of valve operating systems, specifically in terms of motor operated valves. Such parameters to be measured, for example, are torque on a rotating valve stem, motor power and current output, and valve axial thrust or stress. Although these parameters are desired to be measured on valves, different valve and valve operator configurations require different testing equipment for each of the various type valves. Examples of motor operated valve systems are rotating and nonrotating rising stem valves, air operated valves, and quarter turn or butterfly valves.
A torque measuring apparatus for a valve operating system, in particular a butterfly valve, is disclosed in U.S. Pat. No. 4,787,245 to Anderson et al., the disclosure of which is incorporated herein by reference. An apparatus for monitoring, measuring and recording rotational forces and torque within a valve actuator comprises a load cell supported between two arm members. One arm member is mounted stationary to the housing of the valve actuator and the other arm is interfaced for rotation with a worm gear within the actuator. The connection between the two arm members, through a load cell, results in the blocked rotation of the worm gear and the development of torque within the actuator, which torque is monitored through the measurement of the rotational force at the load cell and knowledge of the moment arm of the force at the load cell. A time related trace of the torque is provided in comparison and correlation is made to the time traces of other system parameters. The apparatus is attached to the butterfly valve by removing the upper bearing cap and attaching the apparatus to the gear box. The operation of the apparatus is such that a counter rotational force is developed at the load cell from the torque generated in the valve actuator over a period of time, as a result of preventing rotation of the worm gear. The output from the load cell is traced electronically and displayed and are recorded on an analytical device such as an oscilloscope. The apparatus of the '245 patent operates in such a way that the counter rotational force, which resists rotation of the movable torque arm, operates to resist rotation of the worm gear to develop a torque within the valve operating system.
The worm shaft of a butterfly valve is mounted within the housing by means of a pair of radial bearings disposed on the shaft on opposite sides of the worm. These bearings rotationally mount the shaft within the housing. Although these bearings provide the required radial support for the shaft, it has recently been discovered that these bearings experience significant axial thrust loading during operation of the butterfly valve. It has also been discovered that the axial loading on these bearings exceeds the thrust design rating for the radial bearings. This has resulted in extensive repair and replacement of the radial bearings in prior art butterfly valve operating systems.
What is needed then is a device to measure axial thrust on the radial bearings in a butterfly valve which would typically be experienced under normal operating conditions. It is therefore an object of the present invention to provide a method and apparatus for measuring the axial thrust or stress developed on a valve operating system.
It is a further object of the present invention to provide a calibrated apparatus and method for determining the stress developed on a butterfly valve stem over the full stroke of its operation.
It is still a further object of the present invention to test butterfly valves under differential pressure, or actual operating, conditions to evaluate the overall performance of the valve.